Atherosclerosis is a vascular disease characterized by a modification of the walls of blood-carrying vessels. Such modifications, when they occur at discrete locations or pockets of diseased vessels, are referred to as plaques. Certain types of plaques are associated with acute events such as stroke or myocardial infarction. These plaques are referred to as “vulnerable plaques.” A vulnerable plaque typically includes a lipid-containing pool of necrotic debris separated from the blood by a thin fibrous cap. In response to elevated intraluminal pressure or vasospasm, the fibrous cap can become disrupted, exposing the contents of the plaque to the flowing blood. The resulting thrombus can lead to ischemia or to the shedding of emboli.
One method of locating vulnerable plaque is to peer through the arterial wall with infrared light. To do so, one inserts a catheter through the lumen of the artery. The catheter includes a delivery fiber that sends infrared light to a delivery mirror. Infrared light reflects off the delivery mirror toward a spot on the arterial wall. Some of this infrared light penetrates the wall, scatters off structures within the arterial wall, and re-enters the lumen. This re-entrant light falls on a collection mirror, which then guides it to a collection fiber. The collection mirror and the delivery mirror are separated from each other by a gap. Because the catheter must be narrow enough to fit through blood vessels, the collection mirror and the delivery mirror are typically separated in the axial direction.
To a great extent, the separation between the delivery mirror and the collection mirror controls the depth from which most of the light gathered by the collection mirror is scattered. To gather more light from scattered from deep within the wall, one increases the gap between the collection mirror and the delivery mirror.
The collection mirror and the delivery mirror are mounted in a rigid housing at the distal tip of the catheter. To enable the catheter to negotiate sharp turns, it is desirable for the rigid housing to be as short as possible. This places an upper limit on the extent of the gap between the two mirrors, and hence an upper limit on the depth from which scattered light can be gathered.